I may be breaking from the conventional thinking on this one. I like to be proactive on my tip maintenance. Since I've been using layered tips (and I don't plan to go back to singles), I try to replace mine when I start to notice the difference in the "feel" of my hit. This usually does happen near "dime width" anyway, but has happened closer to "nickel width". It might be due to the glue affecting the layers, or lack of layers and acting more like a single. Anyway, its definitely a long time between tips for layered, than for singles on how often I play. Additionally, I usually use the hard ones, so this makes it even longer. So to restate, I judge it by how it feels rather than some determined thickness. BTW: I've had tried some of the cheaper layered tips and had layers peel away. Poor gluing process I suppose. Obviously, this caused me to replace the tip than to keep playing with a tip that was falling apart. When I was using singles more, I would replace often, but they were cheap anyway. Too much trouble, and that's more the reason why I went with the hard layered. They just last longer, and I've gotten used to the feel.